Phototrophic organisms are microbes, e.g. in the form of micro-organisms, which can use light directly as an energy source for their metabolism. For example, phototrophic organisms include certain plants, mosses, micro-algae, macro-algae, cyanobacteria and purple bacteria.
It may be desirable for different use purposes to be able to produce biomass, for example in the form of algae, in large quantities and inexpensively. For example, such biomass can be used for creating alternative biofuels, e.g. for the transport sector.
So-called bioreactors are used in order to be able to create biomass on an industrial scale. A bioreactor is a plant for producing organisms outside of their natural environment and within an artificial technical environment. So-called photobioreactors are used in order to cultivate phototrophic organisms. A photobioreactor provides the phototrophic organisms with both light and CO2 and a suitable nutrient solution, if appropriate, so that the phototrophic organisms can correspondingly create biomass.
In general, both open and closed systems are known for photobioreactors. Each of these types of photobioreactors has certain advantages and disadvantages.
In open photobioreactor systems, sometimes also refers to as open ponds, phototrophic organisms are cultivated in a controlled manner in open reservoirs or ponds. In this case, for the most part, a nutrient solution or culture suspension, which contains all of the required nutrients for the relevant organism and CO2, is conveyed in a circuit and illuminated for the most part directly by the sun from the open surface.
Possible advantages of such open photobioreactor systems are a relatively small technical outlay and low power consumption.
However, illumination solely by means of the upwardly open surface entails that only small volumes can be supplied with sufficient light. For the most part, light can only penetrate to a depth of a few centimetres into a nutrient solution to which organisms have been added. The depth of such open photobioreactor systems is therefore generally limited to 20 to 30 cm. The low average light influx leads to low areal growth rates. Thus, a large area must be provided for open photobioreactor systems. As a result, costs are increased considerably for such photobioreactors, particularly in densely populated regions.
In addition, pronounced evaporation and therefore salinity effects may result at the exposed surface. Furthermore, a considerable quantity of CO2 can also diffuse into the atmosphere via the exposed surface. Conversely, contaminants may enter an open photobioreactor via the exposed surface, contaminate the photobioreactor and therefore jeopardise product purity. Furthermore, any heating or cooling of such open photobioreactor systems that may be required is difficult to design. In the case of illumination exclusively with sunlight, a daytime dependence also results, deeper layers often only being illuminated unsatisfactorily, whereas directly at the surface of the open system very high illumination intensities may occur which may, where appropriate, lead to what is known as photoinhibition.
The sum of the mentioned disadvantages or limiting boundary conditions can in particular lead to it often only being possible to use open photobioreactor systems in the form of open ponds all year round in very particular geographical areas.
Closed photobioreactor systems were developed in order to reduce an influence of environmental conditions on the one hand and to achieve a higher yield during the cultivation of phototrophic organisms on the other hand. In such closed systems, a nutrient solution is conveyed through a closed circuit together with the organisms and for the most part is illuminated from outside.
For example, in a pipe photobioreactor, glass or plastic pipes are combined to form a closed circuit and the organisms enclosed therein are supplied with nutrients and CO2 by means of a central unit which for example can contain suitable pumps and sensors.
Closed photobioreactors generally allow a high degree of process control, because the organisms and the surrounding nutrient solution can be heated or cooled well in the closed system, a pH value can be monitored and adjusted if necessary, and additional light can be provided. The closed systems allow a high productivity for a low area requirement, because for example a plurality of closed systems can be arranged above one another or pipes of one system can run in the vertical direction and can thereby be illuminated from all sides. However, shadow effects are always to be expected. In addition, high product purity with low contamination, low evaporation and low electromagnetic interference (EMC) is also possible.
However, technical outlay and corresponding plant investment costs when building complex closed photobioreactors are generally very high compared to open systems.
A multiplicity of technical solutions has already been developed to increase the efficiency of photobioreactors. A measure for the efficiency of a photobioreactor can here be understood to be the quantity of necessary resources, such as for example energy to be supplied in the form of light and/or electricity, area to be provided, nutrients to be provided, etc. in relation to the yield of the photobioreactor in the form of biomass with quantities as high as possible of energy chemically stored therein.
For example, a photobioreactor with rotationally oscillating light sources was described in EP 2 520 642 A1.